Difficulties in achieving precise color plane alignments have hindered development of multi-color laser printers which employ single pass color printing processes. Subimages derived from color image planes must be precisely positioned, relative to each other, or else substantial image degradation results. For example, a subimage misalignment that exceeds about 50 microns produces a detectable degradation in print quality.
Alignment of subimages is difficult to achieve in single pass color printers because precise alignment of the multiple imaging sources is required. Such alignments are subject to change with temperature variations, consumable servicing, printer handling, etc.
Various methods have been proposed to reduce color plane alignment errors in single pass color printers. U.S. Pat. No. 5,287,162 to de Jong et al. describes a method and apparatus for correction of color alignment errors in such a printer. deJong et al. print plural chevrons on an intermediate photoreceptor belt or on a media sheet carried by a copy sheet conveyor. In order to achieve correction values for color alignment errors, de Jong et al. employ plural sensors, one for each color chevron that is printed and sense the relative positions of the chevrons. To achieve proper alignment correction values, each detector and its control circuitry is required to determine a centroid of each arm of a chevron being sensed.
U.S. Pat. No. 5,339,150 to Hubble, III et al. describes a mark detection circuit for a multi-color, single pass, electrophotographic printer, wherein alignment marks are employed to achieve color plane subimage alignment. In one embodiment, Hubble, III et al. use four LED print bars to form a composite color image on a media sheet. A photosensor is placed beneath each print bar and a narrow target line is formed on the belt surface a few scan lines before the start of an exposure frame. The center of the target line is detected by each sensor which produces a corresponding detection signal. More specifically, the system includes multiple sensors placed at each print bar to detect the passage of alignment marks produced by the first print bar. An output signal is generated at each of the three downstream print bars, with the signals being utilized to commence image exposure sequence operations in synchronism with the first image exposure.
In another embodiment, Hubble, III et al enable skew alignment adjustments by forming marks on opposite sides of the photoreceptor, detecting the center of each mark and making adjustments of the position of the downstream print bars, based on detected time differences between opposed marks.
As indicated above, both de Jong et al. and Hubble, III et al. require multiple sensors to enable image alignment in a multicolor printer. Such multiple sensors, and the control circuitry associated with each sensor, add to the cost of the printer. Further, both de Jong et al. and Hubble, III et al. apply their respective marks to either a photoreceptor that is used as an intermediate carrier or directly to print media, the latter requiring a special feed of the print media through the printer to achieve an image alignment action.
It is an object of this invention to provide an improved system and method for subimage color plane alignment in a single pass, color printer.
It is another object of this invention to provide an improved system for subimage color plane alignment in a laser printer, wherein only two alignment mark sensors are required.
It is a further object of this invention to provide an improved method for subimage color plane alignment in a single pass laser printer, wherein such alignment is enabled by the printing of alignment marks directly on a media sheet-carrying belt, obviating the need for use of an intermediate transfer medium.